At one time, mine and tunnel openings were protected and supported largely by the aid of wooden timbers extending both in an upright position and also across the roof of the tunnel. Such wooden timbers consumed an undue amount of space, particularly when upright, while the strength thereof was difficult to measure and seldom accurately predictable. Also, such timbers were frequently subject to deterioration, such as due to the effects of seepage water. A limited useage has been made of steel rods to support the roof and ribs of mine openings, by what has been referred to as "pinning". This consisted of pushing a rich mortar of sand and cement into a borehole, so that when a steel rod or pin was driven in, the mortar would set to form a bond between the rod and the hole. Later, an epoxy resin has been substituted for the sand and cement mortar. However, such cementing has not been able to produce the results desired.
It is reported that, initially, mine roof bolt anchorage was by means of a bolt threaded at one end and slotted at the other; then thrust into a borehole until a wedge inserted in the slot engaged the inner end of the borehole and the rod then driven against the wedge to split the sides of the slotted ends apart and grip the sides of the borehole. However, this was an awkward and tedious operation, due to the likelihood of damage to the bolt threads when driving the bolt inward and to the necessity for drilling the borehole to the exact depth necessary for the length of the bolt.
By midway of the present century, a number of torque set, retractable type anchors had been devised, which were usable with headed bolts and were set by retraction, rather than by being driven into the borehole. Although varied in appearance and form, all of such anchors have in common the mode of expansion by which a threaded conical, tapered or triangular plug is drawn through an outer leaf-like shell to stress a contact area of the shell-borehole interface. The extent of the resultant expansion, as intended to transfer bolt load to the sides of the borehole, is necessarily limited due to the inherent rigidity of any wedge or like triangular plug. Consequently, the loads which the previous anchors will sustain in some rock structures are limited and there is still the possibility of anchorage slippage failures. Rock bolts under tension may be used with anchors in a borehole drilled into the roof of a tunnel, for instance, for a distance sufficient to pass beyond a fractured area or a sufficient distance to provide a beam effect, i.e. clamp together a sufficient height of rock strata to produce a beam which bridges across the roof. Rock bolt anchors also may be located to take advantage of natural forces inherent in "the arch", which scientifically is defined as a means of spanning an opening by resolving downward pressures into horizontal to diagonal thrust.
More recently, resin grouted bolts have been developed, in which two plastic bags of epoxy resin components are pushed abead of a bolt, conventionally a normal or sheared reinforcing bar having serrations, into a borehole drilled to a predetermined depth until the bags reach the inner end of the borehole. Then, as the bar is thrust toward the end of the borehole, the bags containing the resin components are ruptured and the bar rotated to cause the resin components to be mixed. Rotation of the bar, after a selected time period, is stopped, then full machine thrust into the bore is applied, as at 5,000 lbs., and held until the resin reaches the gel stage, as at an elapsed time of 30 seconds. After final setting of the resin mixture, the desired load on the bolt is applied at the rock face. Informational Report No. 1033 of the Mining Enforcement and Safety Administration of the U.S. Department of the Interior (1976) and entitled "Comparative Evaluation of Conventional and Resin Bolting Systems" indicates that fast setting resins which attain 90% strength in 2-5 minutes are desirable. The principal relevance of resin grouted bolts is that they are rotated.
Among the objects of this invention are to provide a bolt anchor which can be anchored solidly and can substantially eliminate slippage, as by multiple-point expansive pressure over an extended circumferential area of the borehole; to provide such an anchor adjustable for selection of a predetermined area of borehole engagement; to provide such a bolt anchor which will normally produce a holding strength approaching the tensile strength of the bolt utilized; to provide such a bolt anchor which will anchor effectively in both hard rock and softer rock; to provide such an anchor with a structured safety stop, against anchor creep or slippage of increasing cam pressure, like a chock used in blocking a wheel; to provide such an anchor which may be placed at substantially any desired position in a borehole; to provide such an anchor which has a plurality of separate contact areas; to provide such an anchor which is adapted for use in different rock structures; to provide such an anchor which may be made in more than one form; to provide such forms which enable ecenomies to be secured, depending on the relative costs of casting and other types of forming; and to provide such an anchor which is efficient and effective in use.